How do you call a C/C++ function from Java? 1

[cpjust]

Hi,
I have a C++ DLL, .lib & .h file.
I need to call the C++ functions that are in the DLL from my Java program.

I've heard about JNI and a native keyword in Java, but I haven't been able to find any comprehensive examples...

For simplicity, lets assume I want to call the following WinAPI function:

LONG RegOpenKeyEx( HKEY  hKey,
                LPCTSTR  lpSubKey,
                  DWORD  ulOptions,
                 REGSAM  samDesired,
                  PHKEY  phkResult );

Header: Declared in Winreg.h; include Windows.h.
Library: Use Advapi32.lib.

What's the easiest way to call this function from a Java program?

I guess the most confusing thing for me is how to represent C++ pointers & references in Java?
In Java, Strings are classes, but in C/C++ they are pointers to arrays of bytes. Also, phkResult is an output parameter passed as a pointer; would that cause any problems?

 

[sedj]

You cannot map Java to existing C/C++ code/functions.

If you need Java to utilise an existing C/C++ API, then
you must use JNI to provide a wrapper around that native functionality.

For example, in "JNI C/C++" a Java String object maps to a C/C++ type of "jstring" - which, in C/C++ can be then converted to a char* .

Read some tutorials. Its actually quite simple - as long as you are not passing odd structures/classes through the calls and keep it all basically primitive data typed).

See :

http://java.sun.com/products/jdk/faq/jnifaq.html

http://www.javaworld.com/javaworld/jw-10-1999/jw-10-jni.html

http://java.sun.com/j2se/1.4.2/docs/guide/jni/spec/jniTOC.html

--------------------------------------------------
Free Java/J2EE Database Connection Pooling Software

http://www.primrose.org.uk

 

[cpjust]

I wanted to create a Java wrapper around a set of C++ API's, and now I need to create a C++ wrapper around the C++ API's first. Great... :-(
Thanks for the links though. They look like they should help.

 

[Diancecht]

Another example:

http://java.sun.com/developer/onlineTraining/Programming/JDCBook/jniexamp.html

Cheers,
Dian